To present the invention, the standpoint adopted is that of the field of fiber optic telecommunications and the component described is a filter making it possible to isolate a single wavelength, often called “channel” from among all those that travel around an optical network. This is achieved by placing in the light beam a filter whose passband is chosen in such a way as to best transmit the channel at the targeted wavelength and its entire modulation, and to stop the other channels.
The invention is of course not limited to the field of fiber optic telecommunications nor to a filter placed between two optical fibers. Other optical components such as for example polarizing crystals may call for this configuration between two optical fibers.
It is particularly advantageous to make filters that can be tuned via an electrical voltage, that can thus be adjusted to the desired channel, and to change channel at will as required. This idea has been developed and repeated by numerous authors. It usually relies on a Fabry-Pérot cavity of variable thickness, various techniques being used to make the mirrors of the cavity.
One technique is described in the article by A. Spisser et al., “Highly Selective 1.55 micrometer InP/airgap micromachined Fabry-Perot filter for optical communications” in Electronics Letters, No. 34(5), pages 453-454, 1998. Other embodiments have been proposed, using micromachined silicon, and alloys based on gallium arsenide.
In all the configurations, and in particular in that described by Spisser et al., who make the mirrors of the cavity by virtue of InP/air “Bragg mirrors”, thin membranes of small diameter (of the order of from 40 to 200 μm) are involved.
Hence, the diameter of the beam must be compatible with the diameter of the membranes, this necessitating optical means of collimation or focusing. A known means of achieving this focusing is the butt welding to the fiber of a silica bar followed by a graded-index lens. For example, such assemblies are marketed by the company Highwave under the name Gradissimo®, and may provide a Gaussian spot a few tens of μm in diameter. Other companies similarly offer fibers furnished with a lens at one of its ends. This setup is commonly called a lensed fiber.
A simple layout for a filter component is therefore:
lensed optical fiber—filter—lensed optical fiber
To obtain an insertion loss limited to 0.5 dB when a light ray passes from one lensed optical fiber to another entails complying with tolerances on the mutual position of the fibers and of the optical component. An order of magnitude of tolerances to be complied with is as follows:                relative tolerance of positioning of the lensed fibers along a longitudinal axis (z) of the fibers: ±25 μm;        relative tolerance of positioning of the lensed fibers along axes (x, y) perpendicular to the longitudinal axis: ±2 μm;        relative tolerance of angular positioning of the lensed fibers: ±5 mrad.        
To comply with such tolerances, it is possible to introduce light into the entrance fiber, and to optimize the position of the exit fiber by using the optical signal, then to immobilize it. This process, called dynamic alignment, is extremely accurate, but lengthy and hence expensive, since the various degrees of freedom are not absolutely independent in practice.
The invention is aimed at proposing a process and a device that are very simple and make it possible to achieve in a passive manner, that is to say without introducing light into the entrance fiber, a sufficient tolerance for the positioning of two optical fibers and of an optical component, at the price of a small increase in the insertion loss as compared with dynamic alignment.
Many optical fiber based components are mounted by successively threading through a common capillary the fibers, the lenses and an optical component which by construction has been given a cylindrical shape.
In the case of small diameter tunable Fabry-Perot filters such as those consisting of InP membranes, a further difficulty prevents the use of this known technique: the filter must likewise be centered along axes (x, y) perpendicular to the longitudinal axis (z) of the optical fibers and oriented angularly in a very accurate manner with respect to the entrance and exit fibers. Moreover, it is necessary to access the electrical contacts situated on the filter for its tunability.
Nevertheless, it is still desirable to carry out the alignment between the entrance and exit fibers beforehand, so as to have to accurately position the filter only.
One could contemplate making two holes of the outside diameter of the fibers in a block of rigid material in which a location has been fashioned in the middle for the filter.
This leads to absolutely unrealistic machining tolerances (Ø126±1 μm over 10 to 15 mm of length). This process is therefore not applicable.